Most known techniques for measuring the wavelength of an optical signal make use of interference phenomena. For example, one prior wavelength measuring device comprises a Michelson interferometer having a movable mirror in one arm, so that the optical path length of that arm can be varied. An input beam of unknown wavelength is input into the system, along with a reference beam having a known wavelength. The interference fringes for the input and reference beams are detected and counted as the mirror is moved, and the input beam fringe count is compared to the reference beam fringe count. From the ratio of the two counts, it is possible to calculate the wavelength of the input beam. Devices of this type are described in U.S. Pat. Nos. 4,319,843, 4,426,155 and 4,329,055.
A second prior art technique for measuring wavelength is described in U.S. Pat. No. 4,173,442. This approach uses a Fizeau interferometer that is essentially a Fabry-Perot interferometer with a continuously variable thickness, i.e., with a continuously varying free spectral range (FSR). Recording of the spatial information from the interference pattern yields signals that can be analyzed to determine wavelength. A further class of prior art wavemeters makes use of Fabry-Perot cavities for producing interference patterns that have spatial periods related to the wavelength of the light. Examples of systems of this type are shown in U.S. Pat. Nos. 4,172,663 and 4,170,416.